refactor(updater): simplify check for available updates

tools/check_for_upgrade.sh

@@ -58,23 +58,20 @@ function is_update_available() {
   local local_head
   local_head=$(git -C "$ZSH" rev-parse $branch 2>/dev/null) || return 0
 
-  # Get remote HEAD. If we can't get it assume there are updates unless there is no connection:
-  # - curl: 6 (could not resolve) or 7 (could not connect)
-  # - wget: 4 (network unreachable)
-  # - fetch: 1 (no route to host)
-  local remote_head ret
+  # Get remote HEAD. If no suitable command is found assume there are updates
+  # On any other error, skip the update (connection may be down)
+  local remote_head
   remote_head=$(
-    curl -fsSL -H 'Accept: application/vnd.github.v3.sha' $api_url 2>/dev/null || {
-      [[ $? -eq 6 || $? -eq 7 ]] && exit 1
-    } || wget -O- --header='Accept: application/vnd.github.v3.sha' $api_url 2>/dev/null || {
-      [[ $? -eq 4 ]] && exit 1
-    } || HTTP_ACCEPT='Accept: application/vnd.github.v3.sha' fetch -o - $api_url 2>/dev/null || {
-      [[ $? -eq 1 ]] && exit 1
-    } || exit 0
-  )
-
-  # If can't fetch remote HEAD, return exit code
-  ret=$?; [[ -n "$remote_head" ]] || return $ret
+    if (( ${+commands[curl]} )); then
+      curl -fsSL -H 'Accept: application/vnd.github.v3.sha' $api_url 2>/dev/null
+    elif (( ${+commands[wget]} )); then
+      wget -O- --header='Accept: application/vnd.github.v3.sha' $api_url 2>/dev/null
+    elif (( ${+commands[fetch]} )); then
+      HTTP_ACCEPT='Accept: application/vnd.github.v3.sha' fetch -o - $api_url 2>/dev/null
+    else
+      exit 0
+    fi
+  ) || return 1
 
   # Compare local and remote HEADs
   [[ "$local_head" != "$remote_head" ]]